Signaling system



March 13, 1934. w H GRlNsTED 1,951,148

SIGNALING SYSTEM Filed April 27, 1931 2 Sheets-Sheet 1 HI l0 March 13, 1934. w. H. GRINSTED SIGNALING SYSTEM Filed April 27, 1931 2 Sheets-Sheet 2 VLD l atented Mar. 13, 1934 1,951,148 SIGNALING SYSTEM William Herbert Grinsted, Bromley, England, as signer to Siemens Brothers & Company, Lim-' ited, London, England Application April 2'2, 1931, Serial No. 533,138 In Great Britain April 28, 1930 10 Claims.

This invention relates to signaling arrangements in which the signal current is alternating current and the signal circuit may be traversed by currents for .other than signal purposes, which currents may include components of the signalfreguency or frequencies more than one is empl e The invention is accordingly applicable to a telephone system in which, over a circuit which is 19 used for speech purposes, signaling by means of voice frequency current is carried out. In connection with such systems various proposals have been made with a view to preventing the signal receiving apparatus being affected by speech our- 15 rents even if such speech currents include a transient component of the frequency of the signal pare t- In the present invention means are provided for automatically limiting the input to the signal 39 receiving apparatus. These means are so arranged that pure signal current can operate with maximum effect on the receiving apparatus and the effect of current of another frequency or .an mpure current is relatively lessened. In addi- 25 tion to this .the means of the present invention by imposing a limit .on the input is adapted to act asan adjustment for circuits of lower attenuation than the average to overcome difficulties connected the arrival of strong signals of f-reque ciesadjacent to the signaling frequencies.

The limiting apparatus may be directly associated with a circuit tuned to the signal frequency. .Such ,a .circuit presents a minimum or maximum impedance to currents of the signal frequency according .to whether a series arrangement of condenser and inductance is employed or a parallel arrangement.

If current of another frequency or an impure .current is fed .to the tuned circuit, the eifective impedance of the same is altered. This change of effective impedance may be utilized to cause the limiting apparatus to .come into action and the tuned circuit in its control of the parts intended to be operated by a signal becomes comparatively ineffective.

The limiting apparatus may be of a nature dependingoncurrent or voltage for its operation.

In one form apair of two-electrode thermionic valves (diodes) is employed and the tuned circuit is of the rejector type and is in series with the valves. {The valyes are arranged to work arly :at the saturation point. If the arriving current is not .pf i pure signal frequency the tuned cir u P Q WP l -then t m m imped- 55 ance t t .i d ceof the diodes increases to an extent as to pass current not appreciably greater than when current of pure signal frequency arrives.

In another form a bridge circuit of rectifiers is connected in parallel with a tuned circuit of the series type, .a polarized electrolytic cell which may be an ordinary accumulator being connected across the corners of the bridge. If currents of other than the signal frequency arrive, the impedance of the tuned circuit relatively to other parts of the circuit increases and the bridge serves as a low resistance by-pass if the voltage across it due to the relative change of impedance is higher than that when current of pure signal frequency is received.

The tuned circuit may be utilized in various Ways for controlling the parts intended to be operated by the signal current. Tappings from the circuit may be connected to thermionic valves for rectification or amplification before rectificaticn.

Two or more frequencies may be used for signaling purposes and a common limiting apparatus may serve in connection with the tuned circuit for each frequency or a signal involving sevso eral frequencies may be dealt with as will hereinafter be more particularly described.

Reference will now be had to the accompanying drawings which illustrate diagrammatically modes of carrying out the invention.

In Fig. 1 of the drawings the primary winding of transformer T is connected to the circuit over which the signals arrive. This transformer would usually be in the anode circuit of the last amplifying valve from the circuit. One terminal of the secondary winding is connected through resistances B1 and R2 to a circuit tuned to the signal frequency comprising a choke or reactance L and a condenser C in series, and the other terminal is connected to the other end of said tuned circuit at one terminal thereof. The junction point of the inductance and condenser is connected to the grid of the anode bend rectifying thermionic valve V, in the plate circuit of which is a relay R which is shunted by a small 119 condenser C1 to assist rectifying action. GB and HT indicate the grid bias and high tension batteries respectively associated with the valve. Instead of including a relay R in the plate circuit we may utilize the plate currents in well known manner and control a relay by amplified currents obtained from a second valve.

Bridged across the connections from the transformer at points P and Q is a four armed bridge having in each arm a rectifier for instance a dry plate rectifier of the copper-copper oxide type. A single 2 volt accumulator B is shown connected across the corners of the bridge and it will be observed that the rectifiers (shown in conventional manner) prevent any substantial feed to the secondary winding of transformer T, and also do not substantially shunt the tuned circuit if the voltage between points P and. Q is less than that of the cell B.

The operation of the arrangement is substantially as follows. Relay R will operate if the potential at the junction point S swings sufficiently. If pure signal current is fed to the transformer T the tuned circuit formed by inductance L and condenser C will resonate and the current flowing is in the first place determined by the values of the resistance R1, R2, the resistance of L, and the pressure difference across the terminals of the secondary winding of T. A comparatively large voltage will be found across the condenser C in such resonance conditions and cause operation of relay R. The voltage between the points P and Q will depend on the current flowing and the resistances of R2 and the inductance L. Provided this does not exceed the voltage of the cell B there is no appreciable shunting of the tuned circuit by the cell, which is the case when signal current at its normal strength is received. The resistance R2 is shown merely to indicate that a resistance may be necessary to ensure that the bridge arrangement forms an effective shunt such as in the case where the connections to the corners of the bridge are tappings from a potentiometer fed by a cell such as B.

If the received current is of a frequency other than the signal frequency or is impure in that it contains frequencies other than the signal frequency pure resonance does not occur in the tuned circuit and the impedance of the circuit to the right of the bridge (R2, L, C) increases relatively to the effective resistance of R1. In these circumstances (assuming that the current is of the order of magnitude of the normal. signal current) the bridge becomes effective as a shunt, reducing the potential applied to the tuned circuit and only a fraction of the current supplied by the secondary of the transformer flows through condenser C whereby the potential swing at point S is insufficient to cause operation of relay R.

The bridge arrangement in general acts as a cut-off to limit the voltage applied to the tuned circuit on the arrival of a strong signal whether of signal frequency or otherwise.

Fig. 2 shows an arrangement in which a current limiting device is employed. In this arrangement two thermionic valves D1 and D2 of the two electrode type are employed, one for each half-wave, and work almost saturated. Condensers C3 and C4 serve as blocks for direct current. The incoming leads 1 and 2 are connected to a transformed such as referred to in connection with Fig. 1.

The tuned circuit here is of the rejector type, the condenser C and inductance L being in parallel and the combination being in series with the diodes D1 and D2. The grid of the valve V is maintained at a suitable potential by the battery GB so as to work as an anode bend detector. The voltage on the grid varies according to the voltage across the tuned circuit.

If pure signal current is fed to the circuit over leads 1 and 2 the tuned circuit offers a very high impedance and the voltages applied to the plates D1 and D2 are adjusted so that the diodes are on the verge of saturation with normal current of signal frequency. The voltage applied to the grid is comparatively high on account of the high impedance of the tuned circuit and causes operation of relay R. If current of another frequency or impure current is fed to the circuit the impedance offered by the tuned circuit is less but the current will not be correspondingly greater owing to the saturation of the diodes. The voltage across the tuned circuit will be lower and relay R will not operate.

Fig. 3 shows arrangements adapted to be used when several separate frequencies are used for signaling purposes, the same limiting apparatus serving for the several frequencies.

VLD represents the bridge and resistances R1 and R2 the same as those of Fig. l.

Tuned circuits for four frequencies are shown connected in parallel, the inductances being L1, L2, L3 and L4. Separate tappings 4, 5, 6 and '7 from the junction points in the tuned circuits are led to the grids of four valves arranged as in Fig. 1. A common grid bias battery GB is used for the valves, the filament connections being made at 3.

Each of the tuned circuits presents a low impedance to current of the frequency to which it is tuned and forms a low impedance shunt to the other circuits at the time current of such frequency is supplied. The arrangement behaves, in respect of current of a frequency for which any one of the circuits is tuned, exactly as the arrangement of Fig. 1.

Fig. 4 shows an arrangement employing the current limiting device of Fig. 2 which is shown at CLD. It is adapted to respond to any one of four frequencies for each of which a tuned circuit is provided, the reference characters L1, L2, L3, L4 denoting the inductances respectively of the four tuned circuits. These inductances are inductively coupled with the leads 4, 5, 6, 7 which lead to the grids of valves in the same way as shown in Fig. 2. A common grid bias battery is employed and the filaments of the. valves are connected to the junction 3.

Each of the series tuned circuits presents a high impedance to current of the frequency for which it is tuned and a low impedance to other frequencies. Consequently any of four relays corresponding to relay R of Fig. 2 can be operated by feeding current of the appropriate frequency over leads 1 and 2.

Fig. 5 shows an arrangement in which a complex current composed of currents of three different frequencies can cause the signal receiving parts to operate. The leads 10 and 11 are connected to the grid and filament of a valve such as V in Fig. 2.

What I claim as new and desire to secure by Letters Patent is:

1. In a telephone system in which signals are transmitted by current of a particular voice frequency over the talking path, receiving apparatus responsive only when the input to said apparatus is a predetermined value at signal frequency, and means for limiting the total input to said apparatus to said predetermined value so that said apparatus will not respond to actual voice currents containing a signal frequency component.

2. In a telephone system in which signals are transmitted over the talking path by current of a particular voice frequency, signal receiving apparatus comprising a circuit tuned to the signal frequency, a vacuum tube operated when the sigrial frequency voltage applied to the tuned circuit reaches a predetermined value, means controlled by said tube for repeating the signals, and means for limiting the total voltage impressed on said tuned circuit to said predetermined value whereby the signal frequency component of voice currents present during conversation is insufiicient to effect the operation of said tube.

3. In a telephone system in which signals are transmitted over the talking path by current of a particular voice frequency, receiving apparatus comprising a circuit tuned to the signal frequency, a vacuum tube for repeating received signals, said tube controlled by the signal frequency voltage applied across said tuned circuit, and a shunt for said tuned circuit comprising four rectifiers arranged in a Wheatstone bridge with a source of potential connected across a diagonal of the bridge, said shunt being effective to limit the voltage across the tuned circuit to the potential of said source and thereby preventing the control of said tube by voice currents including a signal frequency component.

4. In a signaling system, a line over which signals composed of current of a particular frequency are transmitted and over which other currents which may include the signal frequency are transmitted for other purposes, signal receiving apparatus, a tuned circuit coupled to said line, said circuit tuned to the signal frequency and effective to operate said receiving apparatus when current of signal frequency of a certain minimum strength is received, and a device associated with said tuned circuit for limiting the input thereto to a predetermined maximum value so that the signal frequency component of said other currents is insufficient to cause said tuned circuit to operate said receiving apparatus.

5. In a signaling system, a line over which signals composed of current of a particular frequency are transmitted and over which other currents which may include the signal frequency are transmitted for other purposes, signal receiving apparatus, a tuned circuit coupled to said line, said circuit tuned to the signal frequency and efiective to operate said apparatus when the signal frequency voltage applied thereto is greater than a predetermined value, and a circuit connected in parallel with said tuned circuit for limiting the voltage applied to said tuned circuit to a value slightly greater than said predetermined value, thereby preventing the operation of said receiving apparatus by said other currents which include a signal frequency component.

6. In a signaling system, a line over which signals composed of current of a particular frequency are transmitted and over which other currents which may include the signal frequency are transmitted for other purposes, signal receiving apparatus, a tuned circuit coupled to said line, said circuit tuned to the signal frequency and effective to operate said apparatus when the signal frequency voltage applied thereto is of predetermined value, and a device connected in multiple with said tuned circuit for limiting the voltage applied to said tuned circuit to said predetermined value, said device comprising four rectifiers arranged in a Wheatstone bridge and a source of potential connected in a diagonal of said bridge.

'7. In a signaling system, a line over which signals composed of current of a particular frequency are transmitted and over which other currents which may include the signal frequency are transmitted for other purposes, signal receiving apparatus, a tuned circuit coupled to said line, said circuit tuned to offer a high impedance to current of the signal frequency and said current of the signal frequency effective to operate said receiving apparatus when the voltage at the terminals of the impedance is a predetermined value, and a current limiting device in series with said tuned circuit, said device limiting the current through the tuned circuit to a predetermined value, thereby reducing the potential across the tuned circuit and preventing the operation of the receiving apparatus by currents other than signal frequency.

8. In a signaling system, a line over which signals composed of current of a particular frequency are transmitted, signal receiving apparatus, a tuned circuit which offers a high impedance tocurrent of said signal frequency, the voltage across said circuit controlling said signal receiving apparatus, and a current limiting device in series with said tuned circuit to limit the current to a predetermined value, thereby reducing the potential across said circuit and preventing the operation of said receiving apparatus when currents of other than signal frequency are present, said device comprising two two-electrode valves connected in parallel and operating at saturation when the signal current is of normal strength.

In a selective signaling system, a plurality of tuned circuits connected in multiple, each circult tuned to a different signaling frequency, separate receiving apparatus controlled by each circuit when the correct signal frequency at a predetermined value of voltage is impressed on the circuit, and a common voltage limiting device for limiting the voltage impressed on said circuits to said predetermined value thus preventing the operation of any receiving apparatus when current other than at a single signaling frequency is received.

10. In a selective signaling system, a plurality of tuned circuits connected in series, each circuit tuned to present a high impedance to current of a different signaling frequency, separate receiving apparatus controlled by each circuit when the voltage across the associated circuit reaches a predetermined value due to received current of a predetermined value at the correct signaling frequency, and a current limiting device connected in series with said tuned circuits, said device limiting the current through the tuned circuits to said predetermined value and preventing the operation of any receiving apparatus when the received current includes other than a single signaling frequency component.

WILLIAM HERBERT GRINSTED. 

